496 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



made by Warner (1914), Wager (1914), Ewart (1915), and WiUstatter and StoU (1918). 

 We also mentioned that Warner, Wager, and Ewart suggested that formaldehyde may 

 be formed by the phoioxidation of chlorophyll. A similar conclusion could be drawn 

 from the experiments of Osterhout (1918), who reported that, when filter paper colored 

 by a chlorophyll extract in carbon tetrachloride was exposed to sunhght in an airtight 

 bell jar until it was bleached, the presence of an aldehyde could be discovered in an 

 open dish of water placed beside the paper. 



However, Willstatter and StoU (1918) denied the formation of formaldehyde by 

 photoxidation of pure chlorophyll preparations. Possibly, aldehydes can be formed by 

 a sensitized oxidation of methanol or ethanol by illuminated chlorophyll (cf. page 466). 



The experiments of Baur and coworkers (e. g., Baur and Fricker 1937, Baur, Gloor, 

 and Kiinzler 1938, and Baur and NiggU 1943), in which formaldehyde was allegedly 

 produced by reduction of chlorophyll in the presence of certain "accessory" oxidation- 

 reduction systems, also were discussed and criticized in chapter 4 (pages 90 et seq.). 



Lommel pointed out, in 1871, that the fundamental principle of 

 photochemistry — "light has no chemical effects unless it is absorbed" — 

 requires that chlorophyll should be bleached most rapidly by blue and 

 red light; but other investigators, working with inadequate equipment, 

 arrived at different conclusions. Thus, Sachs asserted, in 1864, that the 

 intensity of bleaching is parallel to the luminosity of light, that is, that 

 yellow and green rays (although comparatively weakly absorbed by 

 chlorophyll) have the strongest effect. This hypothesis was supported 

 by Wiesner (1874), but was discredited by the work of Reinke (1885), 

 Dangeard (1910), and Wurmser (1921), who found that the "photo- 

 chemical sensitivity spectrum" of chlorophyll is (as expected) roughly 

 parallel to its absorption spectrum. However, the quantum efficiency 

 of the bleaching of chlorophyll must not necessarily be the same for all 

 wave lengths, and some observations point to a greater efficiency of blue 

 and violet light, as compared with red light. 



Wurmser (1921) found, for the ratio of the initial decolorization 

 velocities of chlorophyll in acetone (for equal absorbed energies), the 

 values 0.41, 0.056, and 1.34 in red, green, and violet light, respectively. 

 The low value for green light — which is only weakly absorbed by chloro- 

 phyll — is probably unreliable; but the increased sensitivity in the violet 

 may be significant. In explaining it, two facts may be recalled. In the 

 first place, it was mentioned on page 484 that some excited molecules in 

 the state B, reached by absorption of violet light, may undergo a direct 

 photochemical dissociation. In the second place, we noted on page 465 

 that the " oxychlorophyll " obtained by the reaction of chlorophyll with 

 ferric ions is very sensitive to violet light. Thus, a stronger bleaching 

 effect of blue-violet light may be caused both by a specific effect of this 

 light on chlorophyll itself and by its destructive influence on a yellow 

 product of the reversible primary process. 



The quantum yield, y, of the bleaching of chlorophyll is very small: 

 since the "half-time" of bleaching in intense light (in which a molecule 



